Aqueous cleaning formulations are well known in a variety of commercial forms including those for household use such as laundry detergents, automatic dishwashing liquids, carpet shampoos, metal cleaners, floor tile cleaners, etc. Industrial applications for aqueous cleaning solutions are also known. More recently, aqueous formulations have been used to clean electronic circuit assemblies such as printed circuit boards or printed wiring boards. Copending, commonly assigned U.S. application Ser. No. 731,512, filed Jul. 17, 1991 and now abandoned describes such an aqueous cleaning composition for use in removing soldering flux and other residues from electronic circuit assemblies.
The present invention is directed to improvements in aqueous cleaning compositions which are useful in a wide variety of household and industrial cleaning products and which utilize high concentrations of alkaline salts and include at least one organic adjuvant such as a surfactant to improve the cleaning efficiency of the product. A particularly preferred usage of the cleaning compositions of this invention is for cleaning electronic circuit assemblies. It has been discovered that aqueous cleaners such as disclosed in the aforementioned commonly assigned application are effective in removing soldering fluxes from the electronic circuit assemblies such as printed circuit or wiring boards without the corrosive effects of previous highly alkaline circuit board cleaners and are substantially safer for the environment exhibiting lower biological oxygen demands and chemical oxygen demands than formulations currently available. The addition of surfactants and/or anti-foam agents to these lower alkaline aqueous cleaners substantially improves cleaning performance. However, it has become problematic to maintain the organic adjuvants in solution, in particular, when such solution is an aqueous concentrate.
The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichlorethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly caustic solution having a pH of 13 in a batch cleaning process. This method severely oxidizes the solder applied to the circuit board. In Hayes et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and other residues are removed from electronic assemblies by means of terpene compounds in combination with terpene emulsifying surfactants by rinsing in water.
The complete removal of adhesive and other residues also poses a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which can be removed by the compositions and method of the present invention include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
The above-identified commonly assigned U.S. Ser. No. 731,512 is directed to improvements in electronic circuit assembly cleaners and discloses a cleaning composition comprising an aqueous solution of alkaline metal salts, in particular, alkaline metal carbonate, bicarbonate and mixtures of such salts. The addition of at least one organic adjuvant such as a surfactant or antifoam agent to such compositions has been found to effectively increase the cleaning proficiency of such aqueous alkaline salt cleaners. Unfortunately, at high concentrations of the alkaline salts, such as in the aqueous concentrate form in which the cleaning compositions are typically supplied to the end user, the organic adjuvants are not very soluble and tend to precipitate from the solution. It is desirable to supply the cleaning concentrates as uniform solutions so that no mixing is necessary before addition of the concentrates to the wash bath. Moreover, solubility of the organic adjuvants in the aqueous concentrate ensures solubility in the dilute wash bath. Thus, there is a need to supply a means of maintaining uniformity of the concentrate.
It is known, in general, to utilize hydrotropes to maintain the solubility of organic materials such as surfactants in aqueous solution. In some cleaning compositions and, in particular, cleaning compositions utilized to remove solder fluxes from electronic circuit assemblies it is important that the composition be low foaming. Accordingly, it is also important that the addition of a hydrotrope not increase the foaming tendency of the aqueous cleaning composition.